The present invention concerns improved hard-soft ionomeric resin mixtures (or blends) which are particularly well suited for the formulation of the cover composition of a golf ball.
More specifically, the present invention relates to novel golf ball cover compositions comprising blends of hard ionomer resins with recently developed soft ionomer resins made from acrylic acid based polymers. The new compositions of the present invention, when utilized for golf ball construction, particularly the construction of two piece golf balls, produce golf balls exhibiting enhanced travel distance without sacrificing the properties of playability and/or durability.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability, and flight characteristics, various ionomeric resins sold by E.I. DuPont deNemours and Company under the trademark xe2x80x9cSurlyn(copyright)xe2x80x9d and more recently, by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademarks xe2x80x9cEscor(copyright) and the tradename xe2x80x9cIotekxe2x80x9d, have become the materials of choice for the construction of golf ball covers over the traditional xe2x80x9cbalataxe2x80x9d (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability necessary for repetitive play.
Ionomeric resins are generally ionic copolymers of an olefin such as ethylene and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic groups in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. improved durability, etc., for golf ball cover construction over balata. However, the advantages gained in increased durability have been offset to some degree by the decreases produced in playability. This is because although the ionomeric resins are very durable, they tend to be very hard when utilized for golf ball cover construction, and thus lack the degree of softness required to impart the spin necessary to control the ball in flight.
As a result, while there are currently more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcements, etc., a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the improved impact resistance and carrying distance properties produced by the xe2x80x9chardxe2x80x9d ionomeric resins, but also the playability (i.e. xe2x80x9cspinxe2x80x9d) characteristics previously associated with the xe2x80x9csoftxe2x80x9d balata covers, properties which are still desired by the more skilled golfer.
In various attempts to produce such an ideal golf ball, the golfing industry has blended the hard ionomeric resins with a number of softer polymeric materials, such as softer polyurethanes. However, the blends of the hard ionomer resins with the softer polymeric materials have generally been dissatisfactory in that these balls exhibit numerous processing problems. In addition, the balls produced by such a combination are usually short on distance.
In addition, various xe2x80x9chard-soft ionomeric blendsxe2x80x9d, i.e. mixtures of ionomer resins which are significantly different in hardness and/or flexural modulus, have been attempted. However, until the development of the specific blend combination set forth in U.S. Pat. No. 4,884,814, these balls were not particularly commercially viable. In this regard, although the balls produced using the hard-soft ionomer blends exhibited enhanced playability characteristics, they lacked the durability needed for continuous play.
U.S. Pat. No. 4,884,814, the present inventors previous patent, is directed to the finding that if various xe2x80x9chardxe2x80x9d Surlyn(copyright) methacrylic acid based ionomer resins (i.e. those Surlyn(copyright) resins having a hardness of about 60 to 66 on the Shore D scale as measured in accordance with ASTM method D-2240) were blended with a number of specific xe2x80x9csoftxe2x80x9d Surlyn(copyright) methacrylic acid based ionomer resins (i.e. those Surlyn(copyright) resins having a hardness from about 25 to 40 as measured on the Shore D scale) that a golf ball cover composition could be produced that is not only softer than the prior art hard ionomer covers but also exhibits a sufficient degree of durability for repetitive play.
As a result, a golf ball covered in accordance with the xe2x80x9chard-softxe2x80x9d methylacrylic acid based ionomer blends of the ""814 patent, exhibits properties of enhanced playability (i.e. softness and spin) and durability. However, notwithstanding the above, some sacrifice in carrying distance is also exhibited in comparison with the balls produced utilizing the hard Surlyn(copyright) resins.
The present invention is directed to new golf ball cover compositions which exhibit properties of enhanced carrying distance (i.e. possess higher coefficient of restitution values) than the hard-soft ionomer blends set forth in U.S. Pat. No. 4,884,814, without sacrificing characteristics such as playability (i.e. softness and spin) and/or durability. It has been found that these properties can be produced using improved hard-soft ionomer blends containing recently developed acrylic acid based soft ionomer resins.
The present invention is directed to improved cover compositions for golf ball construction and the resulting golf balls produced utilizing the improved cover compositions. The novel golf ball cover compositions of the invention comprise a blend of hard ionomeric resins, preferably acrylic acid based ionomers, and recently developed acrylic acid based soft ionomers. When the cover compositions of the invention are utilized to manufacture golf balls, the golf balls produced thereby, exhibit properties of improved distance without sacrificing playability and/or durability when compared to known hard-soft ionomer blends.
Two of the principal properties involved in the performance of golf balls are resilience and hardness. Resilience is determined by the coefficient of restitution (C.O.R.), the constant xe2x80x9cexe2x80x9d, which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact. As a result, the coefficient of restitution (i.e. xe2x80x9cexe2x80x9d) can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision.
Resilience (C.O.R.), along with additional factors such as clubhead speed, angle of trajectory, and ball configuration (i.e. dimple pattern), generally determines the distance a ball will travel when hit. Since clubhead speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
The coefficient of restitution (C.O.R.) in solid core balls is a function of the composition of the molded core and of the cover.
In balls containing a wound core (i.e. balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings. Although both the core and the cover contribute to the coefficient of restitution, the present invention is directed solely to the coefficient of restitution which is affected by the cover composition.
In this regard, the coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and measuring the ball""s incoming and outgoing velocity electronically. As mentioned above, the coefficient of restitution is the ratio of the outgoing velocity to incoming velocity. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golf Association (U.S.G.A.). Along this line, the U.S.G.A. standards indicate that a xe2x80x9cregulationxe2x80x9d ball cannot have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second. Since the coefficient of restitution of a ball is related to the ball""s initial velocity, it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of softness (i.e. hardness) to produce enhanced playability (i.e. spin, etc.).
The hardness of the ball is the second principal property involved in the performance of a golf ball. The hardness of the ball can affect the playability of the ball on striking and the sound or xe2x80x9cclickxe2x80x9d produced. Hardness is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball""s diameter (i.e. the lower the compression value, the harder the material). As indicated in U.S. Pat. No. 4,674,751, xe2x80x9csofterxe2x80x9d covers permit the accomplished golfer to impart proper spin. This is because the softer covers deform on impact significantly more than balls having xe2x80x9charderxe2x80x9d ionomeric resin covers. As a result, this allows the better player to impart fade, draw, or backspin to the ball thereby enhancing playability. Such properties can be determined by various xe2x80x9cspin rate testsxe2x80x9d, such as the xe2x80x9cnine-ironxe2x80x9d spin rate test set forth below.
Accordingly, the present invention is directed to new hard- soft ionomer blends which produce, upon molding around solid or wound cores to formulate a cover composition, golf balls exhibiting enhanced distance (i.e. resilience) without adversely affecting, and in many instances, improving the ball""s playability (i.e. hardness/softness) and/or durability (i.e. impact resistance, etc.) characteristics.
These and other objects and features of the invention will be apparent from the following description and from the claims.
The present invention relates to improved cover compositions for golf ball construction. More particularly, the present invention is directed to improved blends of hard and soft ionomers, which, when utilized to formulate the cover stock of golf balls, produce golf balls having enhanced properties, including longer distance with similar or improved playability properties (i.e. higher Riehle compression, lower Shore hardness values, etc.), when compared to golf balls produced by the hard-soft ionomer blends of the prior art. The improved properties produced by the hard-soft ionomer blends of the present invention are due to the use of recently developed acrylic acid based soft ionomers, which produce cover compositions having lower flexural modulus and hardness (i.e. enhanced softness) and improved coefficients of restitution when incorporated with the hard ionomer resins indicated below.
The hard (high modulus) ionomers suitable for use in the present invention include those ionomers having a hardness greater than 50 on the Shore D scale as measured in accordance with ASTM method D-2240, and a flexural modulus from about 15,000 to about 70,000 psi as measured in accordance with ASTM method D-790.
The hard ionomer resins utilized to produce the cover compositions are ionic copolymers which are the sodium, zinc, magnesium or lithium salts of the reaction product of an olefin having from 2 to 8 carbon atoms and an unsaturated monocarboxylic acid having from 3 to 8 carbon atoms. The carboxylic acid groups of the copolymer may be totally or partially (i.e. approximately 15-75 percent) neutralized.
Preferably, the hard ionomeric resins are copolymers of ethylene and either acrylic and/or methacrylic acid, with copolymers of ethylene and acrylic acid the most preferred. In addition, two or more types of hard ionomeric resins may be blended into the cover compositions in order to produce the desired properties of the resulting golf balls.
Although the scope of the patent embraces all known hard ionomeric resins falling within the parameters set forth above, only a relatively limited number of these hard ionomeric resins are commercially available. In this regard, the hard ionomeric resins sold by E.I. DuPont de Nemours Company under the trademark xe2x80x9cSurlyn(copyright)xe2x80x9d, and the hard ionomer resins sold by Exxon Corporation under either the trademark xe2x80x9cEscor(copyright)xe2x80x9d or the tradename xe2x80x9cIotekxe2x80x9d are examples of commercially available hard ionomeric resins which may be utilized in the present invention in the particular combinations described in detail below.
The hard ionomeric resins introduced under the designation xe2x80x9cEscor(copyright)xe2x80x9d and now sold under the new designation xe2x80x9cIotekxe2x80x9d, are somewhat similar to the hard ionomeric resins sold under the xe2x80x9cSurlyn(copyright)xe2x80x9d trademark. However, since the xe2x80x9cIotekxe2x80x9d ionomeric resins are sodium or zinc salts of poly(ethylene acrylic acid) and the xe2x80x9cSurlynxe2x80x9d resins are zinc or sodium salts of poly(ethylene methacrylic acid) some distinct differences in properties exist. As more specifically indicated in the data set forth below, the hard Iotek resins (i.e. the acrylic acid based hard ionomer resins) are the more preferred hard resins for use in the present invention. In addition, various blends of xe2x80x9cIotekxe2x80x9d and xe2x80x9cSurlynxe2x80x9d hard ionomeric resins, as well as other available ionomeric resins, may be utilized in the present invention.
Examples of commercially available hard ionomeric resins which may be utilized in the present invention include the hard sodium ionic copolymer sold under the trademark xe2x80x9cSurlyn 8940xe2x80x9d and the hard zinc ionic copolymer sold under the trademark xe2x80x9cSurlyn 9910xe2x80x9d. Surlyn 8940 is a copolymer of ethylene with methacrylic acid with about 15 weight percent acid which is about 29% neutralized with sodium ions. This resin has an average melt flow index of about 2.8. Surlyn 9910 is a copolymer of ethylene and methacrylic acid with about 15 weight percent acid which is about 58% neutralized with zinc ions. The average melt flow index of Surlyn 9910 is about 0.7. The typical properties of Surlyn 9910 and 8940 are set forth below in Table 1. TABLE 1
In addition, examples of the more pertinent acrylic acid based hard ionomer resins suitable for use in the present invention sold under the xe2x80x9cIotekxe2x80x9d tradename by the Exxon Corporation include xe2x80x9cIotek 4000xe2x80x9d (formerly xe2x80x9cEscor 4000xe2x80x9d), xe2x80x9cIotek 4010xe2x80x9d, xe2x80x9cIotek 8000xe2x80x9d (formerly Escor 900), xe2x80x9cIotek 8020xe2x80x9d, and xe2x80x9cIotek 8030xe2x80x9d. The typical properties of the Iotek hard ionomers are set forth below in Table 2.
The critical soft (low modulus) ionomers utilized to formulate the blends of the present invention are acrylic acid based soft ionomers. These soft ionomers may be generally characterized as comprised of sodium or zinc salts of a terpolymer of an olefin having from about 2 to 8 carbon atoms, acrylic acid, and an unsaturated monomer of the acrylate ester class having from 1 to 22 carbon atoms. Preferably, the soft ionomer is a zinc based ionomer made from an acrylic acid base polymer and an unsaturated monomer of the acrylate ester class. The soft (low modulus) ionomers have a hardness from about 20 to about 40 (preferably from about 30 to about 40) as measured on the Shore D scale and a flexural modulus from about 2,000 to about 10,000 psi (preferably from about 3,000 to 7,000 psi) as measured in accordance with ASTM method D-790.
More particularly, the present inventors have discovered that if the new acrylic acid based experimental soft ionomers recently developed by the Exxon Corporation for the Spalding Sports Worldwide, a division of Spalding and Evenflo Companies, Inc., Tampa, Fla., are utilized with the specific hard ionomers described above, in the combinations more clearly defined below and demonstrated in the Examples, not only are improvements in processability and clarity seen, but also the overall combinations, when utilized for golf ball construction, produce golf balls having higher coefficient of restitution values (i.e. longer distance) at equal or softer hardness than golf balls produced by known hard-soft ionomer blends, including the hard-soft ionomer blends utilized to produce Spalding""s current Tour Editions(copyright) ball.
In this regard, the inventors have found that when the new ethylene-acrylic acid based soft ionomer resins recently developed by the Exxon Corporation under the designations xe2x80x9cIotek 7520xe2x80x9d (referred experimentally by differences in neutralization and melt indexes as LDX 195, LDX 196, LDX 218 and LDX 219) and xe2x80x9cIotek 7510xe2x80x9d are combined with the known hard ionomers indicated above, the combination produces higher C.O.R.s at equal or softer hardness, higher melt flow (which corresponds to improved, more efficient molding, i.e. fewer rejects), as well as significant cost savings versus balls produced by known hard-soft ionomer blends due to lower overall raw materials cost and improved yields.
While the exact chemical composition of the resins to be sold by Exxon under the designation Iotek 7520 is considered by Exxon to be confidential and proprietary information, Exxon""s Experimental Product Data sheet lists the following physical properties of this ethylene acrylic acid zinc ionomer developed by Exxon:
In addition, test data collected by the inventors indicates that Iotek 7520 resins have Shore D hardnesses of about 32-36 (per ASTM D-2240), melt flow indexes of 3xc2x10.5 g/10 min (at 190xc2x0 C. per ASTM D-1288), a flexural moduluses of about 2500-3500 psi (per ASTM D-790). Furthermore, testing by an independent testing laboratory by pyrolysis mass spectrometry indicates that the Iotek 7520 resins are generally zinc salts of a terpolymer of ethylene, acrylic acid, and methyl acrylate.
Furthermore, the inventors have found that a newly developed grade of an acrylic acid based soft ionomer available from the Exxon Corporation under the designation Iotek 7510, is also effective, when combined with the hard ionomers indicated above in producing golf ball covers exhibiting higher C.O.R. values at equal or softer hardness than those produced by known hard-soft ionomer blends. In this regard, Iotek 7510 produces has the advantages (i.e. improved flow, higher C.O.R. valves at equal hardness, increased clarity, etc.) produced by the Iotek 7520 resin when compared to the methacrylic acid base soft ionomers known in the art (such as the Surlyn 8625 and the Surlyn 8629 combinations disclosed in U.S. Pat. No. 4,884,814).
In addition, Iotek 7510, when compared to Iotek 7520, produces slightly higher C.O.R. valves at equal softness/hardness due to the Iotek 7510""s higher hardness and neutralization. Similarly, Iotek 7510 produces better release properties (from the mold cavities) due to its slightly higher stiffness and lower flow rate than Iotek 7520. This is important in production where the soft covered balls tend to have lower yields caused by sticking in the molds and subsequent punched pin marks from the knockouts.
According to Exxon, Iotek 7510 is of similar chemical composition as Iotek 7520 (i.e. a zinc salt of a terpolymer of ethylene, acrylic acid, and methyl acrylate) but is more highly neutralized. Based upon FTIR analysis, Iotek 7520 is estimated to be about 30-40 wt.-% neutralized and Iotek 7510 is estimated to be about 40-60 wt.-% neutralized. The typical properties of Iotek 7510 in comparison of those of Iotek 7520 are set forth below:
As mentioned above and more clearly indicated below in the Examples, the recently developed ethylene-acrylic acid based soft ionomers (i.e. the Iotek 7520 and the Iotek 7510 resins) produce, when combined with various hard ionomers, particularly the Escor(copyright) hard ionomers, golf balls exhibiting enhanced properties, including longer distance at similar or softer hardness, over known hard-soft ionomer blends including those set forth in the U.S. Pat. No. 4,884,814 concerning related subject matter. Along this line, while the ""814 patent is directed to various hard-soft Surlyn(copyright) ionomer blends which are similar to those blends utilized to formulate Spalding""s current Tour Edition(copyright) golf ball, the soft Surlyn ionomers (i.e. the Surlyn 8265 and 8269 resins) disclosed therein are of the poly(ethylene-methacrylic acid-butyl acrylate) type. The inventors have now discovered that the use of soft ionomers containing acrylic acid (i.e. the Iotek 7520 and the Iotek 7510 resins), as opposed to methacrylic acid (i.e. the Surlyn 8265 and the 8269 resins), produce a more desirable golf ball in terms of distance and hardness (softness) while maintaining durability when used in combination with various known hard resins.
The superior golf balls of the present invention containing the improved hard-soft ionomer blends can be generally produced from a central core and an outer cover wherein the outer cover is made from a composition comprised of a blend of about 90 to about 10 percent of a hard ionomer of the type indicated above, and about 10 to about 90 percent of an acrylic acid based soft ionomer.
More preferably, it has been found that a golf ball exhibiting properties of enhanced travel distance (i.e. higher C.O.R. values) with similar or improved playability (i.e. softness characteristics) without a sacrifice in durability, can be produced from a core and a cover, wherein the cover is made from a composition comprised of about 70 to 30 percent of a hard ionomer resin, and about 30 to about 70 percent of an acrylic acid based soft ionomer, depending on the degree of softness desired.
Superior results may be achieved when the hard ionomers utilized are sodium or zinc salts of poly(ethylene acrylic acid) such as those sold by Exxon under the Iotek designation, particularly Iotek 4000 and Iotek 8000, and the soft ionomers utilized are the new acrylic acid based soft ionomers recently developed by Exxon under the designations Iotek 7520 and Iotek 7510. The optimal ranges of hard to soft ionomers are from about 60 to about 40 percent hard ionomer and from about 40 to about 60 percent soft ionomer.
Additional materials may also be added to the compositions of the present invention, including dyes (for example, Ultramarine Blue sold by Whitaker, Clark, and Daniels of South Plainsfield, N.J.) (see U.S. Pat. No. 4,679,795), pigments such as titanium dioxide, zinc oxide, barium sulfate and zinc sulfate; UV absorbers; antioxidants; antistatic agents; and stabilizers. Moreover, the cover compositions of the present invention may also contain softening agents, such as plasticizers, processing aids, etc., and reinforcing materials such as glass fibers and inorganic fillers, as long as the desired properties produced by the golf ball covers of the invention are not impaired.
The cover compositions of the present invention may be produced according to conventional melt blending procedures. Generally, the hard ionomer resins are blended with the soft ionomeric resins in a Banbury type mixer, two-roll mill, or extruder prior to molding. The blended composition is then formed into slabs and maintained in such a state until molding is desired. If necessary, further additives such as inorganic fillers, antioxidants, stabilizers, and/or zinc oxide may be added and uniformly mixed before initiation of the molding process.
The golf balls of the present invention can be produced by molding processes currently well known in the golf ball art. Specifically, the golf balls can be produced by injection molding or compression molding the novel cover compositions about wound or solid molded cores to produce a golf ball having a diameter of about 1.680 inches and weighing about 1.620 ounces. The standards for both the diameter and weight of the balls are established by the United States Golf Association (U.S.G.A.). Although both solid core and wound cores can be utilized in the present invention, as a result their lower cost and superior performance, solid molded cores are preferred over wound cores.
Conventional solid cores are typically compression molded from a slug of uncured or lightly cured elastomer composition comprising a high cis content polybutadiene and a metal salt of an xcex1, xcex2ethylenically unsaturated carboxylic acid such as zinc mono or diacrylate or methacrylate. To achieve higher coefficients of restitution in the core, the manufacturer may include a small amount of a metal oxide such as zinc oxide. In addition, larger amounts of metal oxide than are needed to achieve the desired coefficient may be included in order to increase the core weight so that the finished ball more closely approaches the U.S.G.A. upper weight limit of 1.620 ounces. Other materials may be used in the core composition including compatible rubbers or ionomers, and low molecular weight fatty acids such as stearic acid. Free radical initiator catalysts such as peroxides are admixed with the core composition so that on the application of heat and pressure, a complex curing or cross-linking reaction takes place.
The term xe2x80x9csolid coresxe2x80x9d as used herein refers not only to one piece cores but also to those cores having a separate solid layer beneath the cover and above the core as in U.S. Pat. No. 4,431,193, and other multilayer and/or non-wound cores.
Wound cores are generally produced by winding a very large elastic thread around a solid or liquid filled balloon center. The elastic thread is wound around the center to produce a finished core of about 1.4 to 1.6 inches in diameter, generally. Since the core material is not an integral part of the present invention, a detailed discussion concerning the specific types of core materials which may be utilized with the cover compositions of the invention are not specifically set forth herein. In this regard, the cover compositions of the invention may be used in conjunction with any standard golf ball core.
As indicated, the golf balls of the present invention may be produced by forming covers consisting of the compositions of the invention around cores by conventional molding processes. For example, in compression molding, the cover composition is formed via injection at about 380xc2x0 F. to about 450xc2x0 F. into smooth surfaced hemispherical shells which are then positioned around the core in a dimpled golf ball mold and subjected to compression molding at 200-300xc2x0 F. for 2-10 minutes, followed by cooling at 50-70xc2x0 F. for 2-10 minutes, to fuse the shells together to form an unitary ball. In addition, the golf balls may be produced by injection molding, wherein the cover composition is injected directly around the core placed in the center of a golf ball mold for a period of time at a mold temperature of from 50xc2x0 F. to about 100xc2x0 F.. After molding the golf balls produced may undergo various further processing steps such as buffing, painting, and marking.
The resulting golf balls produced from the novel hard-soft ionomeric resin combinations of the present invention exhibit enhanced distance and playability properties over the art without sacrificing durability. This is due to the use of acrylic acid based ionomers as the soft ionomer in the overall cover stock composition.
The present invention is further illustrated by the following examples in which the parts of the specific ingredients are by weight. It is to be understood that the present invention is not limited to the examples, and various changes and modifications may be made in the invention without departing from the spirit and scope thereof.